FBMC transmission with offset quadrature-amplitude modulation (OQAM) is one of the candidate transmission schemes for future wireless systems, e.g. for 5G systems. In contrast to the state of the art, particularly in contrast to the cyclic-prefix orthogonal frequency division multiplexing (CP-OFDM) transmission scheme, the FBMC/OQAM transmission scheme has the advantages of a better control of the out-of-band radio power leakage and of a higher spectral efficiency.
Similar to CP-OFDM, the FBMC/OQAM transmission scheme typically uses scattered pilot symbol structures, i.e. pilot symbol structures that are distributed in the frequency and time domain, in order to dynamically estimate a channel response of a transmission channel at a receiver side. Due to the estimation of the transmission channel, coherent detection, e.g. frequency-domain channel equalization, at the receiver side is possible.
Contrary to CP-OFDM, symbols conveyed with the FBMC/OQAM transmission scheme on each FBMC subcarrier are OQAM modulated. For such OQAM modulated symbols, the orthogonality of the symbols detected at the receiver side is—even under ideal transmission channel conditions—only withheld in the real field domain, i.e.Re{<sm,n,sm′,n′>}=δm,m′δm,m where s is a basis function for a transmission signal expansion, m is subcarrier index, n is a time index, <⋅> is the inner product, and δ is the Kronecker delta function.
Therefore, in order to accurately estimate the (complex) channel response on a certain pilot symbol at the receiver side, the state of the art suggests inserting at the transmitter side an auxiliary pilot symbol into the symbol structure near the main pilot symbol. The auxiliary pilot symbol is predominantly inserted, in order to pre-cancel imaginary intrinsic interference leaked from the neighbouring symbols and induced by the filterbanks. For example, such a proposal was made in the following references [1], [2], and [3]    [1] J. P. Javaudin, D. Lacroix and A. Rouxel, “Pilot-aided channel estimation for OFDM/OQAM”, VTC'03 spring, April 2003.    [2] C. Lélé, “OFDM/OQAM modulation: Channel estimation methods, and applications to Multicarrier CDMA and multi-antenna transmission”, PhD thesis, CNAM, Nov. 18, 2008.    [3] WO 2008/007019
For example, the reference [1] proposes to insert, as also illustrated in FIG. 1 in (a), one auxiliary pilot symbol A8 into the symbol structure 110 arranged around the main pilot symbol P. FIG. 1 shows in (a) a prototype filter radiation map on the left hand side, and the symbol structure 110 on the right hand side. In FIG. 1 the vertical axis of the symbol structure 110 defines subcarriers in a frequency domain, and the horizontal axis of the symbol structure 110 defines symbols in a time domain. In order to pre-cancel the imaginary interference experienced at the position of the main pilot symbol P corresponding to the position “0” in the prototype filter radiation map, which is influenced by the prototype filter responses γ1 . . . γ8, the auxiliary pilot symbol A8 is calculated by
      A    8    =            -                        ∑                      i            =            Ω                          ⁢                                            d              i                        ⁢                          γ              i                                            γ            8                                =                                        ∑                          i              ∈                              Ω                ⁢                \                ⁢                                  {                  7                  }                                                              ⁢                                    d              i                        ⁢                          γ              i                                      +                              d            7                    ⁢                      γ            7                                      γ        8            where d1 . . . d7 are modulated payload symbols of the symbol structure 110, γ1 . . . γ8 are the prototype filter responses at the corresponding positions of the symbols d1 . . . d7 and A8, and Ω={1, 2, . . . , 8}.
However, in some cases it may happen that the transmission channel strongly distorts the reception signal detected at the receiver side, e.g. due to a high delay spread or a high Doppler spread channel, so that the perfect real field orthogonality of the detected symbols is not withheld, and the main pilot symbol P experiences strong interference. This interference can significantly deteriorate the channel estimation performance conducted at the receiver side, and thus may lead to a poor bit error rate (BER) and/or block error rate (BLER) performance.
For instance, when a FBMC transmitter transmits on a 3GPP-ETU (extended urban) transmission channel with a long delay spread, the main pilot symbols P are strongly distorted by the interference, so that the overall transmission performance (in terms of BER/BLER) is substantially decreased.
To alleviate this problem, the state of the art, for example the following reference [4], proposes to iteratively cancel the interference by feeding back detected neighbouring payload symbols. However, this detection method is very complex to implement.    [4] C. Lele., R. Legouable., P. Siohan, “Iterative scattered pilot channel estimation in OFDM/OQAM” Signal Processing Advances in Wireless Communications, 2009. SPAWC '09. IEEE 10th Workshop, vol., no., pp. 176-180, 21-24 Jun. 2009.
The detection method proposed in reference [4] also brings challenges to the practical system design. In particular, the challenges to the practical system design arise, because in some cases pilot symbols common to several users are closely allocated to the dedicated payload symbols. This means that the receiver side must detect/decode the surrounding payload symbols belonging to other users, in order to perform a common channel estimation and synchronization. Moreover, in multiple-input and multiple-output (MIMO) precoding cases, the surrounding payload symbols may be channel-dependently precoded. Thus, other users may suffer from a huge loss in detection accuracy of the pilot symbols, when performing iterative channel estimation.
A state of the art FBMC transmitter comprises typically a OQAM pre-modulator, synthesis filterbank (SFB) with a prototype filter p(t). A state of the art FBMC receiver comprises typically an analysis filterbank (AFB) with a receiving prototype filter g(t) matching the transmitting prototype filter p(t), a channel estimator, a channel equalizer, and a OQAM post-demodulator. The transmitting prototype filter p(t) and the receiving prototype filter g(t) are configurable system parameters.
For example, a state of the art receiver 100 as proposed in reference [3] is shown in FIG. 1 in (b). The receiver 100 is proposed, in order to estimate and cancel the interference at the main pilot symbol position (“0”). The receiver 100 has an OQAM AFB 101, a channel estimator 102, a sub-channel equalizer 103, a detector/decoder 104, and an interference estimator 105. Iterations of the channel estimation based on an estimated interference estimation, which is fed back from the interference estimator 105 to the channel estimator 102, are jointly conducted with all related neighbouring symbols. This, however, not only increases substantially the complexity of the receiver 100, but also introduces error propagation from all wrongly detected symbols.
In summary, the state of the art is very vulnerable to distortions of the transmission channel, in particular when the distortion is caused by a long delay spread. The state of the art also proposes only highly complex systems to address this problem, specifically systems complex in terms of receiver design and an impractical frame design. The disadvantages of the state of the art arise mainly from the condition of OQAM real-field orthogonality and the limited performance of localized FBMC pulse shaping under highly distorted channels.
For instance, it can be analytically proven that for the proposal of reference [3] under the assumption of a high channel distortion, each transmitted pilot symbol suffers strongly from the intrinsic interference in both time and frequency dimensions and hence yields a very poor transmission performance. Especially in the high signal-to-noise-ratio (SNR) region, the interference is significantly stronger than the thermal noise, and significantly deteriorates the transmission performance so that an error floor is exhibited in BER/BLER curves.